The relationship between coding sequences and function in haemoglobin.

Gilbert has suggested that the presence of intervening sequences in DNA, called introns, can speed evolution by allowing novel proteins to be constructed from the pieces of existing ones. This hypothesis further suggests that the coding sequences, called exons, correspond to functional parts of the protein. The most striking example so far is the case of the immunoglobulin gamma heavy chain, where the four polypeptide sequences corresponding to the four coding sequences form structurally and functionally distinct parts of the molecule. The relation between the three coding sequences of the beta globin gene and structure or function is not as obvious, but the central coding sequence does code for the part of the globin chain which forms the haem crevice. To further test the idea that coding sequences correspond to functional units of proteins we consider the relationship between the coding sequences of alpha and beta globin genes and the corresponding parts of the complete, tetrameric haemoglobin molecule.